The reason to stress test is to make sure you have a stable system. Before I knew anything about computers(read before I built the computer I am typing on), my little cousin had built my 1st 2 computers, he works in that field.

The 1st one was a bomb, it was about 11-12 years ago, it worked like a charm and even more, not one BSOD in the 5-6 years I've had it and even after. Even the parts which failed everywhere else (LS-120) were rock solid.
When I got my 2nd computer, it was the total opposite, I liked to say that my computer had lots of AI and that's why it was restarting anytime with no pattern whatsoever, however it was rock stable in games. I can recall myself rushing to get into a game so that the computer would not reboot, after 3-5 reboots, I succeeded. This is worst than annoying, you don't have a clue . Then suddently, god knows why, after roughly 3 years of , it started working "normally", with a BSOD or just random reboot every 2 weeks or month. To this day, I still don't know what was causing the problem, I know the thermal paste was applied in a hurry to sh*t.

So, to answer your question, why do you stress test, because you want to have fun with your computer, you don't want to be annoyed by it, to be worried, to wonder when the next thing will go wrong on you. Furthermore, if you plan to overclock it, those reasons are even more important.

I could explain in my opinion why it is a necessity to be absolutely sure that the systems is 100% stable.

Let's do it...
;)

The worst thing it could append in OC is memory bug, because most of our system don't have the capacity (I mean without proper program) to see if there is any bug. Went your Cpu bug or crash, the system automatically restart to avoid failure. This is a great protection, because went re-booting, the memory is trash out and the system reinitialized to Zero. So the system is protecting himself from residue/trace that might introduce error/corruption or other bad things... But this protection have been implemented into the CPU, not in usual memory!

When overclocking, and don't adjust the voltage and test everything, you might introduce error in the code in your file, game, etc... So your are deteriorating your system, even if it seem stable.

That's why some OC people run a dual system, one for the OC testing and benchmaking and another for the real life purpose, so in that way if you corrupt your windows, you won't scrap the entire systems!

Notice that in Prime 95, there is 3 test and each of them have it's purpose!
And overclocking take time and patience but they usually worth it.

My 2 cents...
;)

NB: If I made any error in this explanation please advice, but I think I got it!